The development of nanotechnologies has enabled continual miniaturization of products in the field of microelectronics and notably micro-electromechanical systems (MEMS). The conventional lithography techniques are now no longer able to meet these constant needs for miniaturization, as they do not allow structures to be produced with dimensions under 60 nm.
It has therefore been necessary to adapt the lithography techniques and create etching masks that make it possible to create ever smaller patterns at high resolution. With block copolymers it is possible to structure the arrangement of the constituent blocks of the copolymers, by phase segregation between the blocks thus forming nanodomains, at scales that may be under 50 nm. Owing to this capacity for self-nano-structuring, the use of block copolymers in the fields of electronics or optoelectronics is now well known.
Among the masks investigated for carrying out nanolithography, films of block copolymers, notably containing at least two blocks, with at least one nonpolar block and at least one polar block, for example based on PS-b-PMMA, appear to be very promising solutions as they make it possible to create patterns with good resolution. For such a block copolymer film to be usable as an etching mask, a block of the copolymer must be selectively removed to create a porous film of the residual block, the patterns of which can subsequently be transferred by etching to an underlying layer. For film of PS-b-PMMA, the PMMA (poly(methyl methacrylate)) is usually removed selectively to create a mask of residual PS (polystyrene).
To create such masks, the nanodomains must be oriented parallel or perpendicular to the surface of the underlying layer. Such structuring of the domains requires particular conditions such as preparation of the surface of the underlying layer, but also the composition of the block copolymer.
Thus, the ratios between the blocks of PS and of PMMA make it possible to control the morphology of the nanodomains, i.e. arrangement of the nanodomains in the form of lamellae, cylinders, or spheres for example, and the molecular weight of each block makes it possible to control the size and the spacing of the blocks, i.e. the period L0 of the block copolymer.
So as to be able to fabricate miniature microelectronic or optoelectronic devices industrially, it becomes crucial to synthesize the block copolymers, forming the masks intended for nanolithography by direct self-assembly, reproducibly, so that each mask meets precise specifications, and notably has nanodomains whose form, size and period are controlled.
Synthesis of the block copolymers containing at least one polar block, such as, for example, PMMA, traditionally requires working in a polar solvent, to avoid precipitation of the polymer during synthesis, which leads to poor control of its architecture.
Synthesis of the block copolymers containing at least one nonpolar block and at least one polar block, such as, for example, of the PS-b-PMMA type, is traditionally carried out by anionic polymerization in THF (tetrahydrofuran) at low temperature, generally at a temperature below −70° C.
The first example of polymerization of styrene in THF at low temperature dates back to 1956. This polymerization, using naphthalenesodium as initiator, took place suitably, without a transfer reaction, as described by M. Swarc in “living polymers”, Nature (London) 1956; 178: 1168.
The document with the title “On the termination reaction in the anionic polymerization of methylmethacrylate in polar solvents”, Eur. Polym, Vol. 20, No. 4, pages 349-355, 1984, investigates the kinetics of the termination reaction, during which the active centers are destroyed, in the anionic polymerization of methyl methacrylate MAM in THF at a temperature less than or equal to −75° C. This document is not, however, interested in control of the synthesis of PS-b-PMMA block copolymers intended to be used as masks in applications of nanolithography by direct self-assembly.
However, synthesis of PS-b-PMMA block copolymers in THF poses problems of industrialization owing to the difficulty of being able to store THF at industrial sites. As this polar solvent is a cyclic ether, once purified it has a tendency to oxidize over time to form potentially explosive peroxides. Moreover, synthesis at low temperature represents a large energy cost. Finally, the use of a polar solvent such as THF, which is a hygroscopic solvent that is difficult to render anhydrous, and an alkyllithium, for example sec-butyllithium, which is a very reactive initiator, makes it difficult to stabilize the synthesis conditions from one batch to another. Consequently, the reproducibility of the PS-b-PMMA copolymers synthesized in THF at low temperature does not appear to be sufficient to be able to use these copolymers as masks for nanolithography by direct self-assembly with a view to fabricating miniature electronic and/or optoelectronic devices.
Routes have been investigated for replacing THF with some other solvent in the polymerization of methyl methacrylate MAM. Thus, the document with the title “Polymerization of Methyl Methacrylate initiated by 1,1-DiphenylHexyl Lithium”, Trans. Faraday Soc., 1965, 61, pages 150-158, investigates the anionic polymerization of MAM in toluene at a temperature between 0° C. and −80° C., using 1,1DPHLi (DiPhenylHexyl Lithium) as initiator of the polymerization reaction. However, this document is not interested, either, in controlling the synthesis of block copolymers containing at least one nonpolar block and at least one polar block, such as, for example, PS-b-PMMA intended to be used as masks in applications of nanolithography by direct self-assembly, with a view to improving its reproducibility.
Furthermore, the applicant investigated, in document EP 0 749 987, the continuous polymerization of meth(acrylic) monomers in a nonpolar solvent, and notably in toluene at −20° C., with an initiator mixed with an alkali metal alcoholate. The initiator/alcoholate mixture and the monomer(s) to be polymerized are mixed, with a flow ratio that is kept constant, in a micro-mixer arranged upstream of a (co)polymerization reactor. The method for continuous polymerization described in this document makes it possible to control the kinetics of the polymerization reaction of meth(acrylic) monomers, which is very quick, so that it makes it possible to reduce the impact of secondary reactions, by consuming the monomer very quickly. This document is not interested in PS-b-PMMA block copolymers intended to be used as masks in applications of nanolithography by direct self-assembly, nor in controlling their synthesis with a view to improving their reproducibility, notably in terms of the ratio between the blocks and the molecular weight of each of the blocks.
Because the PS-b-PMMA block copolymers make it possible to produce masks for nanolithography offering good resolution, the applicant searched for a solution for controlling their synthesis in order to be able to control the morphology of the nanodomains, and notably their shape, their size and their period and thus improve their reproducibility.